1. Field of the Invention
The present invention relates to a scanning electron microscope, and more particularly to a scanning electron microscope suitable for observing the inside of recessed portions of very small uneven patterns formed in a relatively large specimen. Furthermore, the invention relates to a method of displaying cross sectional profiles of uneven patterns formed in the surface of a specimen using the scanning electron microscope.
2. Description of the Prior Art
In recent years, in the field of semiconductor integrated circuits, the size of semiconductor substrates has been increasing, while the size of circuit elements integrated on the substrates has been decreasing. In fact, the size of the circuit elements has been reduced to as small as less than one micron (to the submicron size).
In the manufacture of such semiconductor integrated circuits, it is necessary to frequently inspect very small patterns being formed on semiconductor substrates during various manufacturing processes, such as lithography process and etching process, or after completion of the manufacture. A scanning electron microscope (hereinafter abbreviated as SEM) is an essential instrument to inspect such very small patterns.
An SEM comprises an electron gun for emitting an electron beam, an objective lens for creating a magnetic field in the passage through which the electron beam passes, and a secondary electron detector for detecting secondary electrons generated by the impingement of the electron beam on a specimen, the object to be observed.
FIG. 17B shows a cross section of an objective lens of a prior art SEM. The objective lens has a symmetrical configuration with respect to its axis. For clarity, the axis of symmetry is hereinafter referred to as the axis or the Z-axis, the direction parallel to the axis of symmetry as the axial direction, and the direction perpendicular thereto as the lateral direction.
The objective lens has a ring-shaped coil 54 for creating a magnetic field and a magnetic path 41 for guiding the lines of force of the magnetic field created around the coil 54. The magnetic path 41 has an upper pole piece 41a and a lower pole piece 41b. The upper pole piece 41a includes an axially symmetrical cone-shaped portion the bottom of which is provided with a hole through which an electron beam 43 passes. The lower pole piece 41b is a planar plate symmetrical with respect to the axis, the center of which is provided with a hole through which the electron beam 43 passes. A lens field 55 is formed between the hole in the upper pole piece 41a and the hole in the lower pole piece 41b.
In FIG. 17B, the magnetic field strength in the axial direction is indicated by .vertline.Bz.vertline.. The magnetic field strength in the axial direction varies in the direction of the axis (Z-axis), reaching the maximum between the upper pole piece 41a and the lower pole piece 41b.
After passing through the hole in the upper pole piece 41a, the electron beam 43 is focused by the lens field 55, passes through the hole in the lower pole piece 41b, and impinges on a specimen 42, the object to be observed. The impingement of the beam of electrons (primary electrons) on the specimen 42 causes secondary electrons to be emitted from the surface of the specimen 42. The secondary electrons are detected by a secondary electron detector (not shown) disposed above the upper pole piece 41a. Since an electric field acting in the axial direction is formed in the space between the specimen 42 and the secondary electron detector, a Lorentz force acts upon the secondary electrons because of this electric field and the magnetic field created by the objective lens, causing the secondary electrons to move spirally to impinge on the secondary electron detector.
To increase the signal-to-noise ratio of an image (SEM image) produced by the SEM and obtain a highly resolved, sharply focused image, it is essential to increase the ratio of the number of secondary electrons detected by the secondary electron detector to that of secondary electrons emitted from the specimen (the collection ratio). For this purpose, the specimen should be placed at a position where a high axial magnetic field strength is experienced.
In the SEM of FIG. 17B, one possible method for increasing the collection ratio of the secondary electrons is to place the object as close as possible to a position between the upper pole piece 41a and the lower pole piece 41b where the axial magnetic field strength is the highest. However, this method is not suitable for observing a specimen larger than the hole in the lower pole piece 41b because the lower pole piece 41b interferes. Usually, a semiconductor wafer has a diameter of more than a few inches and cannot be directly placed sufficiently close to the position where the axial magnetic field strength is the highest.
Another possible method for increasing the collection ratio of the secondary electrons is to insert the specimen 42 in a position between the upper pole piece 41a and the lower pole piece 41b where the axial magnetic field strength is the highest. However, with this method, it is only possible to observe a specimen small enough to be inserted between the upper pole piece 41a and the lower pole piece 41b. A wafer having a diameter of more than a few inches cannot be observed by this method unless the wafer is sliced into pieces small enough to be inserted between the upper pole piece 41a and the lower pole piece 41b.
Once sliced into smaller pieces, the wafer can no longer be used in the subsequent manufacturing process and must be scrapped after it is inspected using the SEM. Generally, in a clean room where semiconductor integrated circuits are manufactured, it is forbidden to slice wafers, or to bring sliced wafer pieces from the outside because the cleanness of the clean room must be maintained. This means that when the SEM is installed in a clean room, sliced wafer pieces cannot be observed using the SEM. Therefore, an SEM capable of observing a large wafer without slicing it into smaller pieces is much in need in the field of semiconductor circuit manufacture.
FIG. 17C shows a cross section of an objective lens of another prior art SEM. This objective lens has a construction that allows a relatively large specimen 42 to be inserted between the upper pole piece 41a and the lower pole piece 41b.
However, this SEM has the disadvantage that the axial magnetic field strength decreases because of an increased distance between the upper pole piece 41a and the lower pole piece 41b. Also, since the specimen 42 must be moved to a great degree in the lateral direction in order to observe a desired portion thereof, the lower pole piece 41b is required to have an area a few times greater than that of the specimen 42 to be observed. However, it is virtually not possible to put in practical use an objective lens having such a large lower pole piece 41b.
Still another prior art SEM developed with the aim of increasing the collection ratio of secondary electrons is reported in "The Japan Society of Applied Physics, 1988, Spring". This SEM accomplishes an increased axial magnetic field strength by providing an auxiliary coil in addition to the coil 44 of the conventional objective lens. However, this SEM has the disadvantage of an increased size and big power consumption.
Thus, the prior art SEMs have the disadvantage that it is virtually not possible to provide a strong axial magnetic field for observing a relatively large specimen without cutting it into smaller pieces. Therefore, the problem is that the secondary electrons cannot be detected with a high collection ratio, making it difficult to obtain a highly resolved image with a high signal-to-noise ratio from a large specimen.
In particular, the disadvantage in the prior art SEMs is a low collection ratio of the secondary electrons emitted from recessed portions of uneven patterns formed in a specimen. This causes a problem, for example, that it is difficult to obtain a clear SEM image from the bottom of a submicron contact hole of a semiconductor integrated circuit. This is because under a relatively weak axial magnetic field the secondary electrons emitted from the bottom of the contact hole can easily hit the side walls of the contact hole and are mostly prevented from reaching the secondary electron detector.
FIG. 18 is a schematic diagram illustrating the motion of secondary electrons emitted from the bottom of a contact hole. The cyclotron radius (Larmor radius) of the spiralling motion of a secondary electron varies inversely with the magnetic field strength B applied in the axial direction. Therefore, the smaller the magnetic field strength B in the axial direction, the larger the cyclotron radius becomes, thus increasing the possibility of the secondary electron hitting the side walls of the contact hole while decreasing the possibility (collection ratio) of the secondary electron reaching the secondary electron detector.
The above relationship, said of the contact hole, between the axial magnetic field B and the collection ratio of secondary electrons is also applicable to uneven patterns having a high aspect ratio other than contact holes.
Also, the prior art SEMs are characterized by a long focal length and a great depth of focus. This means that when the surface of a specimen is scanned by the electron beam, it is difficult to obtain information about the uneven patterns formed in the surface of the specimen. Therefore, to obtain a cross sectional profile of uneven patterns in the surface of the specimen, it is required to cut the specimen and scan the cross sectional face with the electron beam. However, for the previously mention reason, cutting the specimen wafer must be avoided in the manufacturing process of semiconductor integrated circuits.